Nearly every criminal case the FBI and Justice Department has so far reviewed since launching the largest-ever investigation of the FBI's crime lab unit has involved flawed forensic testimony, The Washington Post reported Tuesday night.

But the problem-plagued review triggered by a 2012 Post report that FBI lab techs could have exaggerated evidence in wrongful convictions was itself marred by controversy, the newspaper notes.

The FBI combed through 160 cases in the huge probe before it stopped last August – resuming just this month after the Justice Department's inspector general hammered officials for delays.

During the long delays, The Post reported, three defendants were executed and a fourth died on death row – all convicted on possibly flawed forensics.

"I don't know whether history is repeating itself, but clearly the [latest] report doesn't give anyone a sense of confidence that the work of the examiners whose conduct was first publicly questioned in 1997 was reviewed as diligently and promptly as it needed to be," Michael Bromwich, who was inspector general from 1994 to 1999, told The Post.

"Now we are left 18 years [later] with a very unhappy, unsatisfying and disquieting situation, which is far harder to remedy than if the problems had been addressed promptly."

The Post reported that officials last August notified a first wave of defendants in 23 cases, including 14 death-penalty cases, that FBI examiners "exceeded the limits of science" when they linked hair to crime-scene evidence.

But concerned that errors were found in the "vast majority" of cases, the FBI restarted the review, bringing the entire process to a grinding halt, The Post reported, quoting an unnamed government official.

The Justice Department objected in January and the standoff was finally resolved this month, The Post reported.

At the center of the massive review is the 10-member FBI crime lab unit that performed hair examinations for federal and state agencies – and testified in cases nationwide involving murder, rape and other violent felonies, The Post noted.

The government's finding already of widespread problems flies in the face of earlier FBI claims that just one rogue examiner, Michael Malone, was to blame, the newspaper reported.

"I see this as a tip-of-the-iceberg problem," Erin Murphy, a New York University law professor and expert on modern scientific evidence, told The Post.

"It's not as though this is one bad apple, or even that this is a one-bad-apple discipline. There is a long list of disciplines that have exhibited problems, where if you opened up cases you'd see the same kinds of overstated claims and unfounded statements."

The investigation aims to review 2,600 convictions and 45 death-row cases from the 1980s and 1990s before DNA testing of hair samples became common, The Post reported.

After more than two years, the review will have addressed about 10 percent of the questionable convictions, and some two-thirds of questionable death-row cases, The Post reported.

The department is notifying defendants about errors in two more death-penalty cases, and in 134 non-capital cases, over the next month, and will complete evaluations of 98 other cases by early October, including 14 more death-penalty cases, The Post reported.

When The Post first uncovered in 2012 the possible tainted evidence in two convictions, lab chemist Frederic Whitehurst, who first blew the whistle on the FBI laboratory in the mid-1990s, told NBC News he watched colleagues contaminate evidence and, in court, overstate the significance of their hair sample matches.